The prior art has already described materials for coating optical fibers; for example, WO 91/03498 and EP 166 926 describe photocrosslinkable formulations based on modified polybutadiene oligomers.
WO 91/03498 describes a photocrosslinkable formulation based on a urethane-acrylate prepolymer which is obtained by reacting a hydrogenated hydroxytelechelic polybutadiene with a diisocyanate and then with an unsaturated hydroxyl compound, this prepolymer being subsequently mixed with a reactive diluent of the alkyl acrylate type and with photopolymerization initiators.
EP 166 926 describes the same type of formulation, which is photocrosslinkable under UV irradiation and is composed of a urethane-acrylate oligomer obtained by reaction of a hydroxytelechelic polybutadiene, which is first of all chlorinated and then reacted with a diisocyanate and, finally, with an unsaturated hydroxyl compound, of a reactive diluent of the alkyl acrylate type, and of photoinitiators.
These two references therefore claim optical fiber coating formulations which are photocrosslinkable under irradiation in a process of free-radical polymerization. This technique is known to the skilled worker to have the following disadvantages:
risk of incomplete crosslinking owing to the impossibility of "Dark Cure", therefore placing limitations on the cycle times of industrial lines PA1 the presence of odours associated with reactive diluents of the alkyl acrylate type PA1 high polymerization shrinkage, causing losses in adhesion PA1 poor peel performance PA1 inhibition of polymerization by atmospheric oxygen. PA1 compatible with short cycle times PA1 that it shows no inhibition by atmospheric oxygen PA1 that "dark cure" is a possibility PA1 that it gives flexible and adhering coatings PA1 that it is based on formulations devoid of pronounced odor. PA1 3,4-epoxycyclohexylmethyl 3,4,'-epoxycyclohexanecarboxylate PA1 limonene epoxide PA1 cyclohexene epoxide PA1 1,2-epoxydodecane. PA1 triethylene glycol divinyl ether PA1 1,4-butanediol monovinyl ether PA1 1,4-bisvinyloxymethylcyclohexane. PA1 solvents PA1 plasticizers PA1 surfactants for improving the wetting of the coating on the substrate PA1 spreading agents PA1 devolatilizing agents PA1 matting agents PA1 rheological agents PA1 antioxidants, UV stabilizers for protecting the photopolymerized coating but not interfering with the operation of photopolymerization. Care will be taken to exclude from the formulation any alkaline species, which are known to inhibit cationic polymerization.
A very effective coating has now been found for protecting fibers from moisture.